1. Field of the Invention
This invention relates to an apparatus for forming a desired deposited film on carrier, particularly to an apparatus for forming a deposited film of amorphous silicon (hereinafter referred to as a-Si) on a carrier, more particularly to an apparatus for forming an a-Si film on a carrier utilizing a chemical reaction of radicals.
2. Description of the Prior Art
a-Si is excellent in electrical characteristic, photoelectric characteristic, etc., has wide applications such as solar cell, electrophotographic photosensitive material, photosensor, thin film transistor and the like, and thus, is one promising material.
Deposited films of a-Si are generally produced in accordance with a plasma decomposition process utilizing glow discharge, a so-called CVD (chemical vapor deposition) process (such as a thermal CVD process, a homo-CVD process, and the like.
Generally in the plasma decomposition process, a material gas which is monosilane (SiH.sub.4) or its dilution with H.sub.2, Ar, He or the like is decomposed utilizing glow discharge and an a-Si film is formed on a desired carrier with the decomposed gas. This a-Si film has an excellent photoconductivity because the number of uncombined bonds of Si in the film is as small as about 10.sup.15 cm.sup.-3. The film has a dark conductivity of about 10.sup.-9 (.OMEGA.-cm).sup.-1 and a light conductivity of about 10.sup.-5 (.OMEGA.-cm).sup.-1 under the irradiation of a light of 600 nm having a light intensity of 1 mw/cm.sup.2.
However, the plasma decomposition process utilizing glow discharge has the following drawbacks.
1. In a plasma formed under discharge, various ions and radicals are inevitably generated and it is difficult to obtain only a desired radical. Some of the ions and radicals generated, particularly some ions have unfavorable effects on the characteristics of the film thus formed. The larger the electric power applied, the more is the kind of the ions and radicals generated.
2. A faster film deposition speed needs a larger electric power. However, a larger electric power causes a larger film etching and consequently film formation at a high deposition speed is not possible.
3. An inflammable gas such as SiH.sub.4 or the like is used as a material gas and hence there is a possibility of explosion.
In the thermal CVD process, a material gas such as monosilane or the like is thermally decomposed to form a radical and the radical is allowed to adhere to a carrier, whereby is produced on the carrier a deposited film of a-Si or the like. That is, SiH.sub.4 gas, for example, is applied to a carrier heated to about 500.degree. C.; there occurs the following reaction on the surface of the carrier; thereby a SiH.sub.2 radical is generated. ##STR1## It is understood that this SiH.sub.2 radical adheres to the carrier surface and an a-Si film is formed on the carrier. In this process, only a desired radical is formed and ions believed to have adverse effects on film characteristics are not generated. Further, the film thus formed is not etched. Therefore, film formation at a high deposition speed is possible.
However, this process has the following drawback. That is, it is necessary that the carrier is heated to a temperature higher than the decomposition temperature of the material gas (e.g. higher than 450.degree. C. when the material gas is SiH.sub.4); as a result, hydrogen atoms terminating the uncombined bonds of silicon disappear resulting in increased number of uncombined bonds of Si atom; a film having a good photoconductive characteristic can not be obtained.
In the homo-CVD process proposed by Scott et al. of I.B.M. Co. of USA, a material gas such as SiH.sub.4 or the like is decomposed in a gas phase and applied to a carrier cooled to about 300.degree. C., whereby an a-Si film is formed In this process, since a decomposed gas is used, heating of the carrier to temperature higher than the decomposition temperature of the material gas is not necessary and a film of good photoconductive characteristic can be obtained. However, since a radical such as SiH.sub.2 or the like formed by thermal decomposition has a short life of several tens msec, the radical can not be effectively utilized for film formation and accordingly a high film deposition speed can not be attained. Further there is still a problem associated with the use of an inflammable gas such as SiH.sub.4 and the like. Gases such as SiF.sub.4 and the like, containing halogen atoms in the molecule are non-flammable and have no fear of explosion. Further, radicals such as SiF.sub.2 and the like produced from the decomposition of the above gases have a relatively long life. Therefore, the gases such as SiF.sub.4 and the like have been considered to be a preferable material gas for a-Si films according to CVD processes. However, because of low reactivity of radicals produced from these gases, the gases could not be effectively used in a-Si film formation apparatuses employed in conventional CVD processes and hence no good a-Si film could be formed from such gases.